This invention relates to a liquid crystal display cell having special orientation films therein, and more particularly to a liquid crystal display cell having orientation films formed by a water surface spreading method and suitable for use as nematic liquid crystal display devices and ferroelectric liquid crystal display devices, and a process for producing the same.
Liquid crystal display cells have been practically used in various displays. In order to obtain good display quality, it is necessary to orientate liquid crystal molecules uniformly. Orientation films for liquid crystals play such a role. Thus, many developments and studies on orientation films have been made. Heretofore, there have practically been used in liquid crystal display devices inorganic orientation films obtained by oblique vapor deposition of inorganic compounds such as SiO, etc.; organic orientation films obtained by forming an organic polymer film of polyimide or the like, followed by rubbing with a piece of cloth (e.g. Japanese Patent Unexamined Publication Nos. 50-83051, and 51-65960 or U.S. Pat. Nos. 3,834,792 and 3,994,567). Recently, it is proposed to use a film of monomolecular layer or a film of multi-laminated monomolecular layers of polyimide or the like obtained by a so-called Langmuir-Blodgett method (hereinafter referred to as "LB method") as the orientation film in liquid crystal display devices (e.g. Japanese Patent Unexamined Publication Nos. 62-209415, 62-211617 and 62-215928).
The above-mentioned orientation films, however, have many disadvantages, respectively. In the case of the inorganic orientation films formed by the oblique vapor deposition, it is necessary to use a vacuum device such as a vacuum vapor deposition device for forming the orientation films, which results in making producibility in mass production insufficient.
On the other hand, in the case of the organic orientation films, they are remarkably excellent in mass production but have a defect in that it is impossible to coat a polymer film with a uniform film thickness. Further, the rubbing treatment with a piece of cloth causes problems in that static electricity is generated, the surface of orientation film is contaminated, and the like. For example, in the case of a super-twisted liquid crystal display cell (STN) using nematic liquid crystals [e.g. SID International Symposium p. 120-123 (1985)], there take place unevenness of display due to non-uniformity of threshold voltage (Vth) caused by film thickness of orientation film, and non-lighted portions due to breakage of indium tin oxide (ITO) electrodes caused by static electricity. Further, short circuit between electrodes is easily produced. In addition, contamination of the orientation film surface causes non-uniformity of frequency dependence of threshold voltage, which results in causing unevenness of display.
In the case of active matrix liquid crystal display devices, the rubbing treatment produces damage of switching elements of thin film transistors (TFT) or diodes, or badness of lighting due to changes in switching properties. Further, it is difficult to control the load by the substrate as a whole at the time of rubbing. Particularly, in the case of a large liquid crystal display device, injuries are caused by rubbing.
In the case of orientation films formed by the LB method, no problems caused by static electricity are produced, but there is a problem in mass production. That is, a polymer film formed by the LB method is a monomolecular film having a film thickness of about 4 .ANG.. Thus, the film thickness is too thin to hide ITO electrodes: this is not preferable from the viewpoint of display quality. Organic orientation films now practically used require the film thickness of about 500 .ANG. or more from the viewpoint of display quality. But according to the LB method, a film of 500 .ANG. thick can be formed by laminating 125 layers of a single LB film of about 4 .ANG.. Thus, the formation of orientation films by the LB method is not suitable for practical production due to remarkably poor workability.
Further, Japanese Patent Unexamined Publication Nos. 57-40228 and 62-227122 propose to use liquid crystal polymers as orientation films but uniform film thickness cannot be obtained by usual coating methods.